title: Therapeutically activating RB: reestablishing cell cycle control in endocrine therapy-resistant breast cancer.
abstract: The majority of estrogen receptor (ER)-positive breast cancers are treated with endocrine therapy. While this is effective, acquired resistance to therapies targeted against ER is a major clinical challenge. Here, model systems of ER-positive breast cancers with differential susceptibility to endocrine therapy were employed to define common nodes for new therapeutic interventions. These analyses revealed that cell cycle progression is effectively uncoupled from the activity and functional state of ER in these models. In this context, cyclin D1 expression and retinoblastoma tumor suppressor protein (RB) phosphorylation are maintained even with efficient ablation of ER with pure antagonists. These therapy-resistant models recapitulate a key feature of deregulated RB/E2F transcriptional control. Correspondingly, a gene expression signature of RB-dysfunction is associated with luminal B breast cancer, which exhibits a relatively poor response to endocrine therapy. These collective findings suggest that suppression of cyclin D-supported kinase activity and restoration of RB-mediated transcriptional repression could represent a viable therapeutic option in tumors that fail to respond to hormone-based therapies. Consistent with this hypothesis, a highly selective CDK4/6 inhibitor, PD-0332991, was effective at suppressing the proliferation of all hormone refractory models analyzed. Importantly, PD-0332991 led to a stable cell cycle arrest that was fundamentally distinct from those elicited by ER antagonists, and was capable of inducing aspects of cellular senescence in hormone therapy refractory cell populations. These findings underscore the clinical utility of downstream cytostatic therapies in treating tumors that have experienced failure of endocrine therapy.
authors.affiliation: Kimmel Cancer Center, Philadelphia, Pennsylvania 19107, USA.
authors.forename: Chellappagounder
authors.initials: C
authors.lastname: Thangavel
authors.forename: Jeffry L
authors.initials: JL
authors.lastname: Dean
authors.forename: Adam
authors.initials: A
authors.lastname: Ertel
authors.forename: Karen E
authors.initials: KE
authors.lastname: Knudsen
authors.forename: C Marcelo
authors.initials: CM
authors.lastname: Aldaz
authors.forename: Agnieszka K
authors.initials: AK
authors.lastname: Witkiewicz
authors.forename: Robert
authors.initials: R
authors.lastname: Clarke
authors.forename: Erik S
authors.initials: ES
authors.lastname: Knudsen
mesh_terms: D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism*; D002453:Cell Cycle / Q000187:drug effects / Q000502:physiology; D045744:Cell Line, Tumor; D019938:Cyclin D1 / Q000096:biosynthesis / Q000235:genetics; D005260:Female; D020869:Gene Expression Profiling; D006801:Humans; D058990:Molecular Targeted Therapy; D009376:Neoplasms, Hormone-Dependent / Q000188:drug therapy / Q000378:metabolism; D010879:Piperazines / Q000494:pharmacology; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D011725:Pyridines / Q000494:pharmacology; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D011960:Receptors, Estrogen / Q000037:antagonists & inhibitors / Q000378:metabolism*; D016160:Retinoblastoma Protein / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D010879:Piperazines; D047428:Protein Kinase Inhibitors; D011725:Pyridines; D012333:RNA, Messenger; D011960:Receptors, Estrogen; D016160:Retinoblastoma Protein; D019938:Cyclin D1; C500026:palbociclib